The present invention relates generally to the modification and aging of proteins through reaction with glucose and other reducing sugars, such as fructose or ribose and more particularly to the inhibition of nonenzymatic glycation of proteins which often results in formation of advanced glycation endproducts and crosslinks.
An elevated concentration of reducing sugars in the blood and in the intracellular environment results in the nonenzymatic formation of glycation and dehydration condensation complexes known as advanced glycation end-products (AGEs). Nonenzymatic glycation is a complex series of reactions between reducing sugars and amino groups of proteins, lipids, and DNA. These complex products form on free amino groups on proteins, on lipids and on DNA (Bucala and Cerami, 1992; Bucala et al., 1993; Bucala et al., 1984). This phenomenon is called “browning” or a “Maillard” reaction and was discovered early in this century by the food industry (Maillard, 1916). The reaction is initiated with the reversible formation of Schiff's base which undergoes rearrangement to form a stable Amadori product. Both Schiff's base and Amadori product further undergo a series of reactions through dicarbonyl intermediates to form AGEs. The significance of a similar process in biology became evident only after the discovery of the glycosylated hemoglobins and their increased presence in diabetic patients (Rahbar, 1968; Rahbar et al., 1969). In human diabetic patients and in animal models of diabetes, these nonenzymatic reactions are accelerated and cause increased AGE formation and increased glycation of long-lived proteins such as collagen, fibronectin, tubulin, lens crystallin, myelin, laminin and actin, in addition to hemoglobin and albumin, and also of LDL associated lipids and apoprotein. Moreover, brown pigments with spectral and fluorescent properties similar to those of late-stage Maillard products have also been found in vivo in association with several long-lived proteins such as lens crystallin proteins and collagen from aged individuals. An age-related linear increase in pigments was observed in human dura collagen between the ages of 20 to 90 years. AGE modified proteins increase slowly with aging and are thought to contribute to normal tissue remodeling. Their level increases markedly in diabetic patients as a result of sustained high blood sugar levels and lead to tissue damage through a variety of mechanisms including alteration of tissue protein structure and function, stimulation of cellular responses through AGE specific receptors or the generation of reactive oxygen species (ROS) (for a recent review see Boel et al., 1995). The structural and functional integrity of the affected molecules, which often have major roles in cellular functions, become disturbed by these modifications, with severe consequences on affected organs such as kidney, eye, nerve, and micro-vascular functions (Silbiger et al., 1993; Brownlee et al., 1985).
Structural changes on macromolecules by AGEs are known to accumulate under normal circumstances with increasing age. This accumulation is severely accelerated by diabetes and is strongly associated with hyperglycemia. For example, formation of AGE on protein in the subendothelial basement membrane causes extensive cross-link formation which leads to severe structural and functional changes in protein/protein and protein/cell interaction in the vascular wall (Haitoglou et al., 1992; Airaksinen et al., 1993).
Enhanced formation and accumulation of advanced glycation end products (AGEs) have been implicated as a major pathogenesis process leading to diabetic complications, normal aging, atherosclerosis and Alzheimer's disease. This process is accelerated by diabetes and has been postulated to contribute to the development of a range of diabetic complications including nephropathy (Nicholls and Mandel, 1989), retinopathy (Hammes et al., 1991) and neuropathy (Cameron et al., 1992). Particularly, tissue damage to the kidney by AGEs leads to progressive decline in renal function, end-stage renal disease (ESRD) (Makita et al., 1994), and accumulation of low-molecular-weight (LMW) AGE peptides (glycotoxins) (Koschinsky et al., 1997) in the serum of patients with ESRD (Makita et al., 1991). These low molecular weight (LMW)-AGEs can readily form new crosslinks with plasma or tissue components, e.g., low density lipoprotein (LDL) (Bucala et al., 1994) or collagen (Miyata et al., 1993) and accelerate the progression of tissue damage and morbidity in diabetics.
Direct evidence indicating the contribution of AGEs in the progression of diabetic complications in different lesions of the kidneys, the rat lens and in atherosclerosis has been reported (Vlassara et al., 1994; Vlassara et al., 1995; Horie et al., 1997; Matsumoto et al., 1997; Soulis-Liparota et al., 1991; Bucala and Vlassara, 1997; Bucala and Rahbar, 1998; Park et al., 1998). Indeed, the infusion of pre-formed AGEs into healthy rats induces glomerular hypertrophy and mesangial sclerosis, gene expression of matrix proteins and production of growth factors (Brownlee et al., 1991; Vlassara et al., 1995). Several lines of evidence indicate that the increase in reactive carbonyl intermediates (methylglyoxal, glycolaldehyde, glyoxal, 3-deoxyglucosone, malondialdehyde and hydroxynonenal) is the consequence of hyperglycemia in diabetes. “Carbonyl stress” leads to increased modification of proteins and lipids, followed by oxidant stress and tissue damage (Baynes and Thorpe, 1999; Onorato et al., 1998; McLellan et al., 1994). Further studies have revealed that aminoguanidine (AG), an inhibitor of AGE formation, ameliorates tissue impairment of glomeruli and reduces albuminuria in induced diabetic rats (Soulis-Liparota et al., 1991; Itakura et al., 1991). In humans, decreased levels of hemoglobin (Hb)-AGE (Makita et al., 1992) concomitant with amelioration of kidney function as the result of aminoguanidine therapy in diabetic patients, provides more evidence for the importance of AGEs in the pathogenesis of diabetic complications (Bucala and Vlassara, 1997).
The global prevalence of diabetes mellitus, in particular in the United States, afflicting millions of individuals with significant increases of morbidity and mortality, together with the great financial burden for the treatment of diabetic complications in this country, are major incentives to search for and develop drugs with a potential for preventing or treating complications of the disease. So far the mechanisms of hyperglycemia-induced tissue damage in diabetes are not well understood. However, four pathogenic mechanisms have been proposed, including increased polyol pathway activity, activation of specific protein kinase C (PKC) isoforms, formation and accumulation of advanced glycation endproducts, and increased generation of reactive oxygen species (ROS) (Kennedy and Lyons, 1997). Most recent immunohistochemical studies on different tissues from kidneys obtained from ESRD patients (Horie et al., 1997) and diabetic rat lenses (Matsumoto et al., 1997), by using specific antibodies against carboxymethyllysine (CML), pentosidine, the two known glycoxidation products and pyrraline, have localized these AGE components in different lesions of the kidneys and the rat lens, and have provided more evidence in favor of protein-AGE formation in close association with generation of ROS to be major factors in causing permanent and irreversible modification of tissue proteins. Therefore, inhibitors of AGE formation and antioxidants hold promise as effective means of prevention and treatment of diabetic complications.
The Diabetic Control and Complications Trial (DCCT), has identified hyperglycemia as the main risk factor for the development of diabetic complications (The Diabetes Control and Complications Trial Research Group, 1993). Compelling evidence identifies the formation of advanced glycation endproducts as the major pathogenic link between hyperglycemia and the long-term complications of diabetes (Makita et al., 1994; Koschinsky et al., 1997; Makita et al., 1993; Bucala et al., 1994; Bailey et al., 1998).
The reactions between reducing sugars and amino groups of proteins, lipids and DNA undergo a series of reactions through dicarbonyl intermediates to generate advanced glycation endproducts (Bucala and Cerami, 1992; Bucala et al., 1993; Bucala et al., 1984).
In human diabetic patients and in animal models of diabetes, AGE formation and accumulation of long-lived structural proteins and lipoproteins have been reported. Most recent reports indicate that glycation inactivates metabolic enzymes (Yan and Harding, 1999; Kato et al., 2000; Verbeke et al., 2000; O'Harte et al., 2000). The glycation-induced change of immunoglobin G is of particular interest. Reports of glycation of the Fab fragment of IgG in diabetic patients suggest that immune deficiency observed in these patients may be explained by this phenomenon (Lapolla et al., 2000). Furthermore, an association between IgM response to IgG damaged by glycation and disease activity in rheumatoid arthritis has been reported (Lucey et al., 2000). Also, impairment of high-density lipoprotein function by glycation has been described (Hedrick et al., 2000).
Methylglyoxal (MG) has recently received considerable attention as a common mediator and the most reactive dicarbonyl to form AGEs (Phillips and Thomalley, 1993; Beisswenger et al., 1998). It is also a source of reactive oxygen species (ROS) (free radicals) generation in the course of glycation reactions (Yim et al., 1995).
Nature has devised several humoral and cellular defense mechanisms to protect tissues from the deleterious effects of “carbonyl stress” and accumulation of AGEs, e.g., the glyoxylase systems (I and II) and aldose reductase catalyze the detoxification of MG to D-lactate (McLellan et al., 1994). Amadoriases are also a novel class of enzymes found in Aspergillus which catalyze the deglycation of Amadori products (Takahashi et al., 1997). Furthermore, several AGE-receptors have been characterized on the surface membranes of monocytes and on macrophage, endothelial, mesangial and hepatic cells. One of these receptors, RAGE, a member of the immunoglobulin superfamily, has been found to have a wide tissue distribution (Schmidt et al., 1994; Yan et al., 1997). The discovery of various natural defense mechanisms against glycation and AGE formation suggests an important role of AGEs in the pathogenesis of vascular and peripheral nerve damage in diabetes. MG binds to and irreversibly modifies arginine and lysine residues in proteins. MG modified proteins have been shown to be ligands for the AGE receptor (Westwood et al., 1997) indicating that MG modified proteins are analogous (Schalkwijk et al., 1998) to those found in AGEs. Furthermore, glycolaldehyde, a reactive intermediate in AGE formation, generates an active ligand for macrophage scavenger receptor (Nagai et al., 2000). The effects of MG on LDL have been characterized in vivo and in vitro (Bucala et al., 1993).
Lipid peroxidation of polyunsaturated fatty acids (PUFA), such as arachidonate, also yields carbonyl compounds; some are identical to those formed from carbohydrates (Al-Abed et al., 1996), such as MG and GO, and others are characteristic of lipids, such as malondialdehyde (MDA) and 4-hydroxynonenal (HNE) (Requena et al., 1997). The latter two carbonyl compounds produce lipoxidation products (Al-Abed et al., 1996; Requena et al., 1997). A recent report emphasizes the importance of lipid-derived MDA in the cross-linking of modified collagen and in diabetes mellitus (Slatter et al., 2000). A number of AGE compounds, both fluorophores and nonfluorescent, are involved in crosslinking proteins and have been characterized (Baynes and Thorpe, 1999). In addition to glucose derived AGE-protein crosslinks, AGE crosslinking also occurs between tissue proteins and AGE-containing peptide fragments formed from AGE-protein digestion and turnover. These reactive AGE-peptides, now called glycotoxins, are normally cleared by the kidneys. In diabetic patients, these glycotoxins react with the serum proteins and are a source for widespread tissue damage (He et al., 1999). However, detailed information on the chemical nature of the crosslink structures remain unknown. The crosslinking structures characterized to date, on the basis of chemical and spectroscopic analyses, constitute only a small fraction of the AGE crosslinks which occur in vivo, with the major crosslinking structure(s) still unknown. Most recently, a novel acid-labile AGE-structure, N-omega-carboxymethylarginine (CMA), has been identified by enzymatic hydrolysis of collagen. Its concentration was found to be 100 times greater than the concentration of pentosidine (Iijima et al., 2000) and it is assumed to be a major AGE crosslinking structure.
In addition to aging and diabetes, the formation of AGEs has been linked with several other pathological conditions. IgM anti-IgG-AGE appears to be associated with clinical measurements of rheumatoid arthritis activity (Lucey et al., 2000). A correlation between AGEs and rheumatoid arthritis was also made in North American Indians (Newkirk et al., 1998). AGEs are present in brain plaques in Alzheimer's disease and the presence of AGEs may help promote the development of Alzheimer's disease (Durany et al., 1999; Munch et al., 1998; Munch et al., 1997). Uremic patients have elevated levels of serum AGEs compared to age-matched controls (Odani et al., 1999; Dawnay and Millar, 1998). AGEs have also been correlated with neurotoxicity (Kikuchi et al., 1999). AGE proteins have been associated with atherosclerosis in mice (Sano et al., 1999) and with atherosclerosis in persons undergoing hemodialysis (Takayama et al., 1998). A study in which aminoguanidine was fed to rabbits showed that increasing amounts of aminoguanidine led to reduced plaque formation in the aorta thus suggesting that advanced glycation may participate in atherogenesis and raising the possibility that inhibitors of advanced glycation may retard the process (Panagiotopoulos et al., 1998). Significant deposition of N(epsilon)-carboxymethyl lysine (CML), an advanced glycation endproduct, is seen in astrocytic hyaline inclusions in persons with familial amyotrophic lateral sclerosis but is not seen in normal control samples (Kato et al., 1999; Shibata et al., 1999). Cigarette smoking has also been linked to increased accumulation of AGEs on plasma low density lipoprotein, structural proteins in the vascular wall, and the lens proteins of the eye, with some of these effects possibly leading to pathogenesis of atherosclerosis and other diseases associated with tobacco usage (Nicholl and Bucala, 1998). Finally, a study in which aminoguanidine was fed to rats showed that the treatment protected against progressive cardiovascular and renal decline (Li et al., 1996).
The mechanism of the inhibitory effects of aminoguanidine in the cascade of glycosylation events has been investigated. To date, the exact mechanism of AG-mediated inhibition of AGE formation is not completely known. Several lines of in vitro experiments resulted in contrasting conclusions. Briefly, elevated concentrations of reducing sugars cause reactions between carbohydrate carbonyl and protein amino groups leading to:                1. Reversible formation of Schiff's bases followed by        2. Amadori condensation/dehydration products such as 3-deoxyglucason (3-DG), a highly reactive dicarbonyl compound (Kato et al., 1990).        3. Irreversible and highly reactive advanced glycosylation endproducts. Examples of early Amadori products are ketoamines which undergo further condensation reactions to form late AGEs. A number of AGE products have been purified and characterized recently, each one constituting only minor fractions of the in vivo generated AGEs. Examples are pyrraline, pentosidine, carboxymethyl-lysine (CML), carboxyethyl-lysine (CEL), crossline, pyrrolopyridinium, methylglyoxal lysine dimer (MOLD), Arg-Lys imidazole, arginine pyridinium, cypentodine, piperidinedinone enol and alkyl, formyl, diglycosyl-pyrrole (Vlassara, 1994).        
Analysis of glycation products formed in vitro on a synthetic peptide has demonstrated that aminoguanidine does not inhibit formation of early Amadori products (Edelstein and Brownlee, 1992). Similar conclusions were reached by analysis of glycation products formed on BSA (Requena et al., 1993). In both experiments AGE formation was strongly inhibited by AG as analyzed by fluorescence measurements and by mass spectral analysis. The mass spectral analysis did not detect peptide complexes with molecular mass corresponding to an incorporation of AG in the complex. Detailed mechanistic studies using NMR, mass spectroscopy and X-ray diffraction have shown that aminoguani dine reacts with AGE precursor 3-DG to form 3-amino-5- and 3-amino-6-substituted triazines (Hirsch et al., 1992). In contrast, other experiments using labeled 14C-AG with lens proteins suggest that AG becomes bound to the proteins and also reacts with the active aldose form of free sugars (Harding, 1990).
Several other potential drug candidates as AGE inhibitors have been reported. These studies evaluated the agent's ability to inhibit AGE formation and AGE-protein crosslinking compared to that of aminoguanidine (AG) through in vitro and in vivo evaluations (Nakamura et al., 1997; Kochakian et al., 1996). A recent breakthrough in this field is the discovery of a compound, N-phenacylthiazolium bromide (PTB), which selectively cleaves AGE-derived protein crosslinks in vitro and in vivo (Vasan et al., 1996; Ulrich and Zhang, 1997). The pharmacological ability to break irreversible AGE-mediated protein crosslinking offers potential therapeutic use.
It is well documented that early pharmaceutical intervention against the long-term consequences of hyperglycemia-induced crosslinking prevent the development of severe late complications of diabetes. The development of nontoxic and highly effective drugs that completely stop glucose-mediated crosslinking in the tissues and body fluids is a highly desirable goal. The prototype of the pharmaceutical compounds investigated both in vitro and in vivo to intervene with the formation of AGEs on proteins is aminoguanidine (AG), a small hydrazine-like compound (Brownlee et al., 1986). However, a number of other compounds were found to have such an inhibitory effect on AGE formation. Examples are D-lysine. (Sensi et al., 1993), desferrioxamine (Takagi et al., 1995), D-penicillamine (McPherson et al., 1988), thiamine pyrophosphate and pyridoxamine (Booth et al., 1997) which have no structural similarities to aminoguanidme.
Clinical trials of AG as the first drug candidate intended to inhibit AGE formation are in progress (Corbett et al., 1992). A number of hydrazine-like and non-hydrazine compounds have been investigated. So far AG has been found to be the most useful with fewer side effects than other tested compounds of the prior art. AG is also a well known selective inhibitor of nitric oxide (NO) and can also have antioxidant effects (Tilton et al., 1993).
A number of other potential drug candidates to be used as AGE inhibitors have been discovered recently and evaluated both in vitro and in vivo (Nakamura et al., 1997; Soulis et al., 1997). While the success in studies with aminoguanidine and similar compounds is promising, the need to develop additional inhibitors of AGEs continues to exist in order to broaden the availability and the scope of this activity and therapeutic utility.